Petroleum fuels such as residual fuel oils contain large amounts of impurities which result in corrosive deposits in the equipment. For example, crude oil usually contains 1-500 ppm of vanadium in the form of a porphyrin complex depending on the source. Because of its origin as a concentrate from the refining process, residual oil contains several times more vanadium than the crude from which it was derived. The combustion of these vanadium-containing fuels produces very corrosive deposits which can destroy a metal part, such as a gas turbine blade, in a matter of hours.
The presence of sodium in fuel can also have catastrophic consequences. For example, in maritime use the sodium level can be increased because of the introduction of sodium chloride through the air intake and contamination of the fuel by sea water. During combustion, the sodium can react with sulfur in the fuel to form a sulfate which is deposited in turbine parts.
Overbased detergents, e.g., overbased alkaline metal or alkaline-earth metal compounds, are well known additives for lubricating oil compositions and petroleum fuels. These detergents perform a variety of functions including anti-corrosion, deposit control, acid scavenger functions and in general comprise overbased metal compounds complexed with an organic dispersant. For example, overbased magnesium compounds complexed with sulfonate and carboxylate dispersants, have long been used as anti-corrosion and acidic neutralization additives for lubricating oils and greases, anti-corrosion and acidic neutralization additives during the combustion of fuels such as residual fuel, pulverized sulfur-containing coal, corrosion inhibitors in fuels containing vanadium etc. The addition of overbased magnesium detergents to, for example, boiler fuels or gas turbine fuels, is known to reduce corrosion, presumably by forming magnesium complexes with the vanadium or sodium.
Overbased metal detergents are also added to lubricating oils to prevent or remove deposits of oil-insoluble sludge, varnish, carbon and lead compounds which otherwise form on internal combustion engine parts and for combating severe rust conditions which may be encountered during shipping or storage of machinery or exposure to out-door weather. Detergent additives for automotive and diesel engine oils also react chemically with the highly acidic by-products of combustion that find their way into the lubricating oil system.
Often, overbased metal additives are added as a dispersion in an appropriate carrier, in the case of lubricants and fuels, a high boiling liquid hydrocarbon is often used. Obviously the dispersion must be stable during storage and the overbased metal must stay well dispersed in the lubricant or fuel.
A variety of parameters will affect the stability and activity of these dispersions such as the dispersants and carriers employed, particle size of the solid components, and the relationship between metal and dispersant. The process by which the overbased metal compounds and complexes are prepared will greatly influence the actual physical make up and properties of the overbased metal dispersion, impacting particle size and distribution of the metal compound throughout the dispersion, the viscosity and stability of the dispersion, the amount of the metal within the dispersion etc.
U.S. Pat. No. 4,163,728, incorporated herein in its entirety by reference, discloses stable, fluid magnesium-containing dispersions prepared by the high temperature decomposition of magnesium salts of carboxylic acids to MgO in a dispersant-containing fluid. In the process, Mg(OH)2, an organic carboxylic acid or sulfonic acid surfactant such as naphthenic acid, acetic acid and water are heated in a high boiling hydrocarbon to temperatures up to 350° C., which is above the decomposition point of magnesium acetate, 323° C. It is believed that magnesium acetate is formed in situ and decomposes at the high temperatures used. Water is also removed at the elevated temperatures.
U.S. Pat. No. 4,293,429, incorporated herein in its entirety by reference, discloses a variation of U.S. Pat. No. 4,163,728 which begins with MgO instead of Mg(OH)2. In the process, the bulk MgO is converted to magnesium acetate which forms suspended MgO particles of less than 5 microns, and preferably less that 1 micron. Thus, the coarse MgO particles are converted into a dispersion of stabilized micro MgO particulates. It is also disclosed that similar processes using lower temperatures fail to provide the fine particle size MgO. Dispersions with 1-32% magnesium are disclosed and stable dispersions with 19.5% magnesium are exemplified. However, the use of the high boiling hydrocarbon solvent can lead to thick, viscous reaction mixtures making appropriate mixing difficult.
U.S. Pat. No. 4,056,479, incorporated herein in its entirety by reference, discloses a fuel additive for reducing sediment in vanadium-containing fuels comprising a magnesium-alkoxide-carbonate complex in combination with an oil soluble sulfonate and a carboxylate and/or phenate dispersing agent. While the additive of U.S. Pat. No. 4,056,479 has a magnesium content of about 12.5% to about 14.6%, it also tends to have undesirably high viscosities.
U.S. Pat. No. 4,129,589, incorporated herein in its entirety by reference, discloses a process for preparing an over-based oil-soluble magnesium salt of a sulfonic acid by contacting carbon dioxide gas with a mixture comprising an oil-soluble magnesium salt of a sulfonic acid, magnesium oxide, a promoter system comprising a carboxylic acid of 1 to 5 carbons, water, optionally a low MW alcohol and an inert solvent for lowering the viscosity of said mixture to facilitate mixing. The products of U.S. Pat. No. 4,129,589 had acceptably low viscosity and the use of the diluent provides for good mixing and reproducible reaction conditions, but the magnesium content was typically 9-10% and no more than 14%.
U.S. Pat. No. 4,931,164, incorporated herein in its entirety by reference, discloses that treating a low (up to about 1% by weight) asphaltene, low aromatic hydrocarbon liquid with an overbased magnesium sulfonate reduced limited asphaltene fouling. However, in fuel oils, such as residual fuel oils, containing both high asphaltenes (at least more than 1%, and generally at least 3 to 4% by weight) and highly overbased magnesium sulfonates would, under certain conditions, particularly with water present, produce deposits or sediment containing both magnesium and asphaltenes which could plug fuel filters.
U.S. Pat. No. 6,197,075, incorporated herein in its entirety by reference, discloses an overbased magnesium sulfonate, carboxylate or phenate product containing at least 14% and up to about 18% by weight of magnesium, and a succinic anhydride and lower carboxylic acid co-promoter reaction product, useful as a deposit control additive for residual fuel oils and turbine fuels, particularly those containing high asphaltenes without clogging filters and which also reduces vanadium caused corrosion in the turbine. The process for preparing the overbased magnesium product comprises contacting a mixture of i) a sulfonic acid, phenol or carboxylic acid or salt thereof, ii) a magnesium oxide, iii) a co-promoter comprising a lower carboxylic acid, a lower alcohol, a succinic anhydride and water, and iv) a solvent and/or oil, with an acidic gas such as carbon dioxide at 50° F. up to the reflux temperature of the mixture to overbase the reaction mixture. The succinic anhydride may be added prior to, during or post carbonation.
The overbased metal compositions described above and elsewhere are best described as products by process as there is typically no simple chemical formula which adequately correlates to the essential material makeup and the physical properties of the product. Often, the molecular structures of the metal complexes are not fully known and are not a critical aspect of the invention. For example, two compositions containing compounds with the same chemical formula in the same amounts and differing only by the manner in which they were prepared can have very different physical properties.
Attempts to modify known procedures to obtain overbased detergents with certain desired characteristics of the final dispersion, e.g., high metal content or low viscosity, have met with unforeseen drawbacks. For example, attempts to facilitate mixing during preparation and obtain a lower viscosity product by adding a diluent solvent to the mixture of MgO, dispersant, water, acetic acid and high boiling hydrocarbon of U.S. Pat. No. 4,293,429, and then heating as described in the Examples therein, lead on many occasions to the formation of a gel and not the desired free flowing dispersion. Thus, a new, readily controlled and reproducible process for preparing stable overbased magnesium dispersions with high levels of magnesium and usable viscosities as an additive in fuels and lubricating oils is desirable.